1. Field of the Invention
The present invention relates to a method of manufacturing a long tube having a small diameter for the manufacture of a tube having an outside diameter of 40 mm or less and a length of 15 m or more, such as tubes for use in the heat exchange in the thermoelectric power plant, the nuclear power plant and the like, requiring a remarkably high quality.
2. Description of Related Art
The heat exchange tube incorporated into heat exchangers, such as, steam generator and water-supply heater, in a thermoelectric power plant and nuclear power plant has an outside diameter of 40 mm or less and it is manufactured by bending a long tube having a length of 20 m or more in a U-letter shape. This U letter-shaped heat exchange tube is subjected to internal eddy-current defect detection for inspection prior to actual use after being incorporated into the heat exchanger and the a periodic inspection is performed after use of tube for a predetermined time. To this end, a defectoscope disclosed in for example Japanese Patent Publication No. 60-621 is used. It goes without saying that the inspection standards of this internal eddy-current defect detection are remarkably severe for the U letter-shaped heat exchange tubes used in the thermoelectric power plant and the nuclear power plant with respect to safety.
A similar internal eddy-current defect detection has been required also for straight long tubes used as materials of the U letter-shaped heat exchange tubes. The results of the defect detection for these straight long tubes are administrated for every one piece of tube so that they may be compared in relation to the results of the defect detection for the heat exchange tubes in the pre-use inspection after they have been formed in a U-letter shape and the results of the periodic defect detection for the in-use heat exchange tubes. It is natural that articles of inferior quality are removed on the basis of the judgment of success or failure in the internal eddy-current defect detection of the long tubes. It has been required also for the successful tubes that the results of the internal eddy-current defect detection are recorded in relation to the positions along the axis of the tube for every one piece of tube.
The straight long tubes, which are materials for the U letter-shaped heat exchange tubes used for the thermoelectric power plant and the nuclear power plant, are manufactured by cold work from mother tubes, such as seamless tubes produced by the hot tube manufacturing method or welded tubes produced by welding. Of the cold work methods, methods accompanied by the reduction in wall-thickness generally include the plug drawing method, the cold rolling method and the mandrel drawing method.
In the plug drawing method, in general, chemically formed coatings and lubricating oils have been used as lubricants. In the case where the chemically formed coatings are used, also the mother tubes are long in the drawing of the long tubes, so that the mother tubes are not sufficiently chemically coated until the depths thereof according to circumstances. In this case, injuries due to jamming are produced at poorly lubricated portions of the drawn long tubes. In addition, in the case where the lubricating oils are used, the lubricating capacity is inferior to that of the chemically formed coatings, so that jamming is apt to occur on the internal side. Accordingly, the plug drawing method is difficult to adopt for the cold work of the long tubes under the usual condition.
In the cold rolling method, although the long tubes can be manufactured without bringing about the jamming, the rolling is conducted by intermittently pushing the mother tubes in the rolling-mill in synchronization with the reciprocal movement of a pair of taper-grooved rolls, so that the dimensional fluctuation in the axial direction of the tube corresponding to this intermittent pushing is unavoidably brought about. Accordingly, the cold rolling method is difficult to adopt for the final cold work of highly accurate long tubes such as the materials of the U letter-shaped heat exchange tubes.
Contrary to the above described methods, the mandrel drawing method is a method in which a mandrel having an outside diameter corresponding to an inside diameter of the long tubes is inserted into the mother tubes to draw out the mother tubes together with the mandrel. The relative movement of the internal surface of the mother tubes relative to the internal tool is smaller than that in the plug drawing method and even the long tubes do not show the jamming on the internal surface thereof. In addition, the drawing is continuously conducted, so that the dimensional fluctuation in the axial direction of the tubes incidental to the cold rolling method is not brought about during the work. Accordingly, this mandrel drawing method has been adopted for the final cold work accompanied by the reduction in wall-thickness of the long tubes for use in the U letter-shaped heat exchange tubes.
However, in this mandrel drawing method, a process of integrally reeling both the long tubes stuck to the mandrel and the mandrel to form a gap therebetween is required in order to separate the long tube from the mandrel after the drawing. As a result, the very small periodical spiral fluctuation in outside diameter is unavoidably brought about in the long tubes by this reeling process. Even though the long tubes, which have been separated from the mandrel, are subjected to the unloaded drawing for uniforming the outside diameter in the axial direction thereof, this very small fluctuation in outside diameter is merely converted into a very small fluctuation in inside diameter. Accordingly, the fluctuation in wall-thickness in the axial direction of the tube can not be solved at all.
If the long tubes showing the very small dimensional fluctuation in the axial direction thereof are subjected to the above described internal eddy-current defect detection having the severe standards, a signal resulting from this very small dimensional fluctuation is detected as a noise. As a result, also in the case where very small defects exist in the long tubes, the defect signals are hidden in the dimensional fluctuation signal, whereby the very small defects are overlooked by the automatic judgment according to circumstances.
The automatic judgment based on an output signal of the defectoscope is impossible and at present an inspector carries out the defect detection by staring at the CRT. When a doubtful signal is output, that portion is subjected to the defect-detection again at a lower speed to detect very small defect signals. As a result, the defect-detecting efficiency is remarkably reduced and the fatigue of the eyes of the inspector is increased.
An apparatus adopting the plug drawing method using a pressurized lubricating oil, which is one kind of the plug drawing method, has been disclosed in Japanese Patent Publication No. 62-39045. This apparatus has been developed by the present applicant and with it, a vessel with a mother tube inserted thereinto is filled with a lubricating oil under a high pressure and the mother tube is drawn out of the vessel under such a condition while it is subjected to the plug drawing. According to this method, the lubricating oil is sufficiently spread over inner and outer surfaces of even the tube, for which the chemically formed coating must be used as the lubricant, that is, this method is superior to the method using the chemically formed coating in jamming-prevention effect.